In the manufacturing of photonic devices, it is often desirable to employ processes that facilitate the monolithic integration of multiple photonic devices on a single substrate. This monolithic integration increases the yield, performance and functionality of the photonic devices and reduces manufacturing cost. The multiple photonic devices can include active photonic devices (e.g., lasers, optical intensity modulators, optical phase modulators, optical switches, optical amplifiers, optical saturable absorbers, optical pulse reconditioners, optical wavelength converters, photon transistors, variable optical attenuators, optical detectors, etc.) and passive photonic devices (e.g., optical waveguides, optical gratings and optical splitters, optical beam couplers, multi-mode interference devices, optical polarizers, optical polarization beam splitters, optical wavelength filters, optical resonators, etc.).
Photonic devices can be made of III-V semiconductor materials. Monolithic integration of multiple photonic devices, however, usually requires that photonic devices with different III-V semiconductor material characteristics (e.g., different bandgap energies) be formed on a single substrate. For example, to monolithically integrate a photonic laser device and a passive optical waveguide device, the photonic laser device must contain active III-V semiconductor materials that emit light at a particular lasing wavelength, while the passive optical waveguide device must contain passive III-V semiconductor materials that are transparent to the light emitted from the photonic laser device. Therefore, the emission/absorption wavelength of the III-V semiconductor materials in the photonic laser device must be different from that of the III-V semiconductor materials in the passive optical waveguide device.
The emission/absorption wavelength of III-V semiconductor materials is determined by their bandgap energy. Thus, monolithically integrating multiple photonic devices on a single substrate requires a process for shifting the bandgap energy (and thus the bandgap wavelength) of a selected substrate portion to a value different from that of another substrate portion. Such a process is often referred to as “bandgap engineering.”
The fabrication of photonic integrated circuits requires a robust process that is capable of high spatial bandgap selectivity across a wafer, while producing little or insignificant change in both optical and electrical properties of the processed material.
Currently, photonic integration methods use epitaxial growth and re-growth. Selective area epitaxy technology, however, adds additional etching and/or manufacturing steps, and thus produces relatively low production yield to the final device. Further, such methods are not planar processes, and integration of multiple photonic elements using these complex techniques is therefore complicated.
Another approach to bandgap engineering is called “Quantum Well Intermixing (QWI).” See for example: U.S. Pat. No. 4,871,690, issued to N. Holonyak, Jr. et al. in October 1989; J. H. Marsh, “Quantum Well Intermixing,” Semicon. Sci. Tech, 8, 1136, 1993; and N. Holonyak, Jr, “Impurity Induced Layer Disordering of Quantum Well Heterostructures: Discovery and Prospects,” IEEE. J. Sel. Topic. Quantum. Electron, 4, 584, 1998. Photonic devices typically have a quantum well (QW) structure that includes a quantum well layer disposed between (i.e., sandwiched between) two barrier layers. The barrier layers have a larger bandgap energy than the quantum well layer and act as a potential barrier to confine electrons in the quantum well layer. QWI shifts the bandgap energy of a quantum well layer in selected areas by intermixing (also called interdiffusing) the atoms between the quantum well layer and its adjacent barrier layers.
The mechanisms of QWI, especially for two phase intermixing processes in a quaternary compound semiconductor, are not fully understood. It is commonly accepted that as point defects or impurities are diffused into the active quantum well structure, which enhances the intermixing rate between the well and the barrier regions, the bandgap of the active well region is shifted to a higher energy.
Several techniques have been found to induce quantum well intermixing (QWI) in III-V quantum well structures. These techniques are impurity induced disordering (IID) (those skilled in the art know that ‘disordering’ is used interchangeably with the terms ‘interdiffusion’ or ‘intermixing’), impurity free vacancy diffusion (or disordering), thin film encapsulant induced group-III vacancy techniques, and laser irradiation induced disordering (including photo-absorption induced disordering and pulsed-photo-absorption induced disordering). There are advantages and disadvantages for each of these post-growth quantum well intermixing techniques.
Impurity Induced Disordering (IID) involves the creation of crystal site vacancies (i.e., point defects). As is well known to those skilled in the art, the atoms in a semiconductor material form a crystal structure and are arranged in a periodic lattice-like fashion. In the case of III-V semiconductor materials, two types of atoms, namely the group III atoms and the group V atoms, are arranged to occupy alternating lattice sites in the crystal structure. These group III and group V atoms exchange electrons and exist as electrically charged ions at the lattice sites. In the case of Aluminum Gallium Arsenide (AlGaAs), the Al and Ga atoms are from group III and the As atom is from group V. In the case of Indium Gallium Arsenide Phosphide (InGaAsP), the In and Ga atoms are from group III and the As and P atoms are from group V. A “crystal site vacancy” is formed when an ion is missing from a lattice site. Such a crystal site vacancy can be formed, for example, by knocking an ion off its site to an “interstitial space” in the crystal structure. A single isolated vacancy or a small group of vacancies is called a “point defect.” A point defect carries the opposite electric charge of the missing ion.
A point defect can move around in the crystal structure when the crystal structure is heated. Heating can cause the atoms in the sample to vibrate violently. Under such thermal vibration, an atom from a lattice site close to the vacancy may move into the vacancy and fill the vacancy (i.e., void). The original site of the atom then forms a new vacancy or point defect, resulting in an effective movement of the point defect from one lattice site to another. Before this type of QWI can happen, point defects are either created at the quantum well structure or migrated to the quantum well structure (for example, to the boundary between a quantum well layer and a barrier layer) via a thermal process.
After the above-mentioned process of having the point defects at the boundary between the quantum well layer and a barrier layer, a subsequent high-temperature crystal annealing step is needed to cause quantum well intermixing (QWI) to occur. Upon high temperature annealing, thermal energy causes some of the point defects in the barrier layer to be filled by atoms from the quantum well layer and some of the point defects in the quantum well layer to be filled by atoms from the barrier layer. In addition, some of the interstitial atoms with opposite charge will also migrate down to meet with some of the vacancies and heal (i.e., fill) the vacancies. On the other hand, some of the vacancies will migrate deep down to the substrate and become diluted out. In short, this annealing process causes an effective exchange (or “intermixing”) of the atoms between the quantum well layer and the barrier layer(s), resulting in a shift of the bandgap energy to a higher value.
In a conventional IID process, the creation of the crystal site vacancies (i.e., point defects) is typically accomplished by introducing impurity atoms/ions into the quantum well structure using a room temperature ion-implantation technique. The ion implantation step is followed by a high temperature anneal step, typically conducted at a temperature of around 800° C. for a GaAs and AlGaAs based quantum well structure, or around 600° C. for an InGaAs and InGaAsP based quantum well structure. In such a conventional IID process, donor ion species (e.g., Si) and acceptor ion species (e.g., Zn) have been utilized. These donor and acceptor ion species are known as shallow-level ion species, because they have a relatively low energy of ionization in III-V material semiconductors. During the high temperature anneal step, some of the point defects created by the implanted atoms/ions, the interstitial ions (i.e., those ions knocked from their lattice sites) and the implanted atoms/ions, will diffuse into the quantum well layer and barrier layers and promote QW intermixing between atoms in the quantum well layer and the barrier layers.
The use of high-energy and/or high dose (i.e., a dose of greater than 1×1015 cm−2) ion implantation in conventional IID processes is known, however, to cause severe damage in the quantum well structure. The more severe damage includes crystal defects known as loops, lines, complexes and clusters. In general, these crystal defects are referred to as “complex defects.”
Conventional ion implantation based IID processes have several drawbacks. These drawbacks include difficulty in producing: (i) a low-loss waveguide photonic device due to free carrier absorption from implanted shallow-level ion species and scattering loss from complex defects induced by the IID process; (ii) photonic devices with controlled electrical characteristics (e.g., a desirable electrical conductivity or pn-junction properties) due to the aforementioned free carriers and complex defects, as well as re-distribution of dopants during the high temperature anneal step; and (iii) a photonic device with a high quality gain layer due to the IID process-induced complex defects, which create carrier recombination centers, resulting in shorter carrier lifetime and lower optical gain.
Active or passive photonic devices such as amplifiers, lasers, detectors, modulators, couplers, transparent waveguides, and many others, require either good electrical conductivity, low waveguide loss, or high optical gain. Another fundamental issue in QW shape modification is the reliability, reproducibility, and simplicity of the spatial selective QWI process. Moreover, QWI tends to damage semiconductor materials, resulting in high waveguide loss and poor material quality. For example, known impurity induced disordering (IID) processes typically use neutral impurity, high-energy ion implantation with a relatively high dose.
The implantation energy tends to be at the MeV level. QW structures intermixed using this technique suffer a certain amount of damage, and hence are characterized by high loss to the passive waveguide sections. See for example J. J. He, et al., ‘Bandgap Shifted InGaAs/InP Quantum Well Waveguides Using MeV Ion Implantation,’ Electronics Letters, vol 31(24), pp 2094, 1995. This material damage shortcoming imposes a limitation on the fabrication of high performance photonic and opto-electronic integrated circuits. Because QWI results in high waveguide loss, this process has historically been used to fabricate devices with relatively short passive waveguide lengths. Further, as the quality of the material is degraded after the intermixing process, this semiconductor material cannot be used to make active devices such as wavelength shifted lasers, modulators or detectors. This reduces the flexibility of device design and the number of devices that can be integrated on a single chip.
In order to address the above process issues, an improved QWI technique based on thermal assisted implantation vacancy induced disordering (TAIVID) has been developed, as disclosed in co-pending U.S. patent application Ser. No. 09/916,701 entitled Method For Shifting the Bandgap Energy of a Quantum Well Layer, which is expressly incorporated herein by reference. This technique is based upon impurity induced disordering (IID) because of its process simplicity and its ability to produce high bandgap and spatial selectivity in InGaAs/InGaAsP quantum well structures. Briefly, this process utilizes in-situ heating (i.e. an elevated temperature) during an ion implantation step to spread out the spatial distribution of the point defects created during ion implantation, followed by an anneal step to induce QWI. To minimize the material damage, this process is carried out using an impurity that is electrically neutral to the material system, at a lower implantation energy, and using a relatively low implant dose. This process reduces the formation of complex crystal defects, and thus retains high material quality with low material loss in the QW structure (e.g. formed of III-V semiconductor materials) after intermixing. Active devices such as wavelength-tuned lasers having been produced using this technique. There was an insignificant change in the threshold current of such lasers relative to As-grown lasers, implying that the material quality remains high after quantum well intermixing using this technique.
While this improved QWI process has been proven to induce a large degree of QWI for InGaAs/InGaAsP lattice matched QW structures, it has been shown to give quite low reproducibility in modifying the bandgap of InGaAs/InGaAsP based strained (compressive and/or tensile) QW structures. There has been a great deal of interest in strained InP-based QW systems for their commercial potential in polarization insensitive devices such as semiconductor optical amplifiers (SOA) and modulators.